This invention relates to metal halide lamps, and more particularly relates to such lamps containing sodium and scandium metal halides having improved lumen output.
Metal halide lamps were introduced commercially in the United States in the early 1960's. In appearance, these metal halide lamps resemble a mercury vapor lamp, having an inner quartz arc tube containing a fill of an arc-sustaining material and surrounded by an outer glass envelope. As in the mercury lamp, the metal halide lamp's arc tube fill includes a rare gas for starting and a quantity of mercury. However, the lamp's emission spectrum is primarily due to the presence in the arc tube fill of one or more metal halides, usually iodides. These metal halides are responsible for a much higher luminous efficacy and color rendering capability of the lamp output than is possible for the mercury vapor lamp. The luminous efficacy, color rendering index and other lamp output characteristics may be varied, depending upon the particular composition of the metal halides in the arc tube fill. It has been said that there are more than 50 different metal halides which may be used alone or in combination in metal halide lamps. However, those combinations which have enjoyed commercial success fall into three categories. An indium-thallium-sodium lamp and a thorium-thallium-sodium lamp were introduced in the mid-1960's by General Electric and Sylvania and were marketed under the trademarks Multivapor and Metalarc, respectively. The thorium-thallium-sodium lamp was later superceded by a scandium-sodium lamp. In addition, a dysprosium-thallium-indium lamp was introduced by Westinghouse and marketed under the trademark BOC. Electric Discharge Lamps, John F. Waymouth, The MIT Press, page 211 (1971).
Of these systems, the thallium-indium-sodium combination appears to offer superior color rendering properties with the sodium contributing to the yellow portion of the spectrum, thallium to the green and indium to the blue. Such a combination is currently enjoying commercial success in Europe. However, in the United States, the sodium scandium lamp has become practically universally accepted, due to its very good luminous efficacy, (typically 85 to 90 lumens per watt) and long operating life (typically 10,000 to 15,000 hours).
Despite the commercial success of metal halide lamps, it is a general objective to further increase the luminous efficacy, color rendering index and operating life of these lamps. For example, in British Patent No. 1,125,063, lithium was added to an indium-thallium-sodium halide lamp in order to improve color rendering by increasing emission in the red portion of the spectrum. In U.S. Pat. No. 3, 521,110, tin, antimony or bismuth is added to an indium-thalliumsodium halide lamp in order to extend operating life.
Because of its acceptable luminous efficacy and superior operating life, and its wide commercial acceptance in the United States, it would be desirable to further improve upon the sodium-scandium metal halide lamp.
Accordingly, it is an object of the invention to increase the luminous efficacy of a sodium scandium metal halide lamp.
It is also an object of the invention to increase the luminous efficacy of such a lamp, while maintaining or even increasing the color rendering index for such a lamp.